426 Prof. Tyudall on the Polymagnet. 



mediately. The apertures 1, 2, 3, 4 are for the reception of pins 

 projecting from a glass case intended to cover the whole ap- 

 paratus. 



When the magnets and central helix are fixed in their places 

 and looked down upon, their appearance is that represented in 

 fig. 4 ; at a and c the tops of the cores arc seen, the moveable 

 masses of soft iron which belong to them being removed ; the 

 two ends of the other electro-magnet bear two such masses, 

 each formed from a parallelopiped 4*5 inches long, 2 inches 

 wide, and l - 25 inch high, having one end bevelled off so as to 

 render it pointed, the other end being suffered to remain flat. 

 The distance between those moveable masses may be varied, or 

 the body to be examined may be suspended either between sur- 

 faces or points, according to the nature of the experiment. The 

 projections of the current reversers upon the horizontal plane are 

 seen right and left. 



Simplicity and efficiency being the objects aimed at, a current 

 reverser was devised which fulfils these conditions. A front 

 view of the instrument is given in fig. 5, and its horizontal pro- 

 jection in fig. 6. Q is the section of a quadrant of wood, which 

 is capable of being turned by the handle HP ; ab is the section 

 of a strip of brass laid on the periphery of the quadrant ; cd is a 

 shorter strip similarly laid on ; between b and c is a gap, formed 

 of the wood of the quadrant itself, or of a piece of ivory or glass 

 inlaid; 5 and s' are two brass springs*, which are shown in the 

 figure to rest upon the strips, of brass ab and cd; M, M' are two 

 clamps secured to the wooden pillars and 0' by screws S 

 which pass up through the latter. The plan below corresponds 

 to the section above. From b, fig. 6, the strip of brass crosses 

 to c', and from c to b', both being insulated from each other at R. 

 Supposing, then, the two clamps M and L to be connected with 

 the two poles of a galvanic battery, the current entering at M 

 would flow along the spring s to b, thence to c', and finally 

 along the spring s' to the clamp L' : in like manner the cur- 

 rent entering at L would attain the clamp M'. In this posi- 

 tion of things the handle of the instrument leans to the left, 

 as in fig. 5. If the current is to be interrupted, this is secured 

 by setting the handle vertical ; for when the handle is in this 

 position, the spring s 1 rests upon the non-conducting surface be, 

 and the circuit is broken. If it be desired to send the current 

 direct from L to L', and from M to M', this is accomplished by 

 causing the handle to lean to the right ; when this is clone, both 

 the springs s, &' rest upon the self-same strip of brass ab, and 

 there is direct metallic communication between L and I/, and 

 between M and M'. This reverser has been tested practically, 

 * Copper, I think, would be better than brass. 



